In the manufacturing of integrated circuits, wafer acceptance tests need to be performed to ensure that the manufactured wafers meet the design specification. Conventionally, test units may be formed in scribe lines and used in the wafer acceptance tests. However, the test units in the scribe lines may not always accurately reflect the performance of intra-die integrated circuits that are formed inside dies. For example, due to the non-uniformity in the wafers, the critical dimensions of polysilicon lines in different portions of a die may be different from each other, and may be from the critical dimensions of polysilicon lines in scribe lines. Accordingly, when the test units are limited in the scribe lines of wafers, the overall uniformity of dies cannot be obtained accurately.
Intra-die test units, which were also known as dose mappers (DOMAs), were thus formed as intra-die test units, so that the intra-die uniformity of the integrated circuits can be measured. However, conventional DOMAs suffer from various limitations.